What's the most common way of forming a question?
Is it by inversion (eg etes-vous instead of vous etes), making a sentence a question (vous etes eux? instead of etes-vous eux?), or using qu'est-ce que/est-ce que?
Est-ce que is more common and I think you can use it almost always, the inversion on the other hand is quite formal.
You can also ask a question just by the intonation without changing its structure, but I would advise not to do that on exams and things like that, just with friends.
When I went to visit a friend I met online, I noticed that her family and her friends used est-ce que a lot more than I thought they would, I was in fact convinced it was more formal than inversion. I was wrong. She also told me that my way of speaking was formal to the point of bit a bit nouveau riche XD
this is a very good answer! I would go on to say, avoid inversion in spoken French altogether as it is really more literary, I don't think I've ever heard someone use an inversion in spoken conversation in all the time I've lived in France, except when I did an 'escape room' in Paris and they used 'qui suis-je' for the riddles